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DIAGNOSTIC /REMEDIAL TEST 11 HEAT TRANSFER This test is one of a series in Introductory Physics made available on the Website of the School of Physics, Monash University, Australia (www.physics.monash.edu.au/community ). This test is NOT for the purposes of assessment. It is to assist you in locating misconceptions and misunderstandings and generally to assist you in your study of Physics. You should work by yourself and at your own pace following the directions given. It is not necessary to attempt the test all at once. You may like to do it bit-by-bit, waiting until you have covered a particular topic in class or in your reading of your text book or you may like to "plunge in " before you begin your study of the topic. Questions are on the left hand (even-numbered) pages. While reading or working on these, keep the right hand (odd-numbered) answer page covered. DO NOT PEEK AT THE ANSWERS ON THE RIGHT HAND PAGE ! The test was compiled by and largely authored by Emeritus Professor Bill Rachinger who would appreciate any comments or suggestions for improvement. These could be sent to him at [email protected] or Emeritus Professor Bill Rachinger, School of Physics Monash University, P.O.Box 27, Vic 3800 Australia Diagrams were produced by Mr Steve McCausland, formerly of Department of Physics, Monash University
Transcript

DIAGNOSTIC /REMEDIAL TEST 11 HEAT TRANSFER

This test is one of a series in Introductory Physics made available on the Website of the School of Physics, Monash University, Australia (www.physics.monash.edu.au/community).

This test is NOT for the purposes of assessment. It is to assist you in locating misconceptions and misunderstandings and generally to assist you in your study of Physics. You should work by yourself and at your own pace following the directions given. It is not necessary to attempt the test all at once. You may like to do it bit-by-bit, waiting until you have covered a particular topic in class or in your reading of your text book or you may like to "plunge in " before you begin your study of the topic.

Questions are on the left hand (even-numbered) pages. While reading or working on

these, keep the right hand (odd-numbered) answer page covered. DO NOT PEEK AT THE ANSWERS ON THE RIGHT HAND PAGE !

The test was compiled by and largely authored by Emeritus Professor Bill Rachinger who would appreciate any comments or suggestions for improvement. These could be sent to him at [email protected] or

Emeritus Professor Bill Rachinger, School of Physics Monash University, P.O.Box 27, Vic 3800 Australia Diagrams were produced by Mr Steve McCausland, formerly of Department of Physics, Monash University

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COVER THE RIGHT HAND PAGE. DO NOT PEEK 1. Heat energy can move from one body to another or from one place to another. The process is known as heat transfer. Some examples of heat transfer are: A. Heat energy travelling from hot coffee through the cup to your hand. B. The ceiling of a room being warmed by hot air rising from a fire. C. Heat energy from the sun reaching your body here on earth. These are all examples of heat transfer but the way in which transfer occurs is different in each case. Four other commonplace examples of heat transfer are listed below. State which of A, B or C above they most closely correspond with. Comment on whether more than one process may be involved. (i) Heat being transferred from the bottom of a heated saucepan and distributing itself

through the water contained in it. ANS (ii) The transfer of heat along the handle of a heated saucepan. ANS (iii) You warm yourself sitting in front of an electric radiator ANS (iv) Your back is warmed by laying on an electric blanket. ANS GO STRAIGHT TO THE NEXT QUESTION. DON'T CHECK YOUR ANSWER YET. WAIT UNTIL YOU ARE INSTRUCTED TO DO SO.

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1. (i) This would be most closely related to B and to a lesser degree to A. (ii) This would be similar to A. (iii) This would be largely similar to C but processes A and B would also play a part. (iv) This is similar to A. If you were unsure of how to categorise your answers, the answers to later questions should help you.

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2. We will now consider the three processes of heat transfer in turn. The first, typified by the transfer of heat through the coffee cup is known as CONDUCTION. If you are holding the coffee cup you are certainly aware that heat energy travels through the cup - This results in an increase in the temperature of your hand. a) Is the energy transfer through the cup accompanied by the transfer of any material? .........(Yes/No). To understand how the energy transfer takes place we must consider what is happening in the coffee cup at an atomic level. Remember an earlier description of the structure of a solid as a regular array of atoms held together by interatomic forces (bonds). Using this idea the regular arrangement of atoms in the wall of the coffee cup can be schematically represented by:

Remember that this is a very rough representation. (The wall of the cup would be about ten million atoms thick and we are representing the bonds between the atoms by "interatomic springs".) b) Before the coffee is poured into the cup the atoms will be: A. Stationary. B. Oscillating more violently near the inside of the cup. C. Oscillating more violently near the outside of the cup. D. Oscillating, on average, by the same amount throughout the thickness. ANS.......... c) A moment after the hot coffee is poured into the cup, the atomic behaviour is described best by A, B, C or D of the previous Question. ANS......... d) A minute after pouring in the coffee the atoms on the outside of the cup will A. oscillate more violently than previously B. oscillate less violently than previously C. oscillate in the same way as before the coffee was poured. ANS........ CHECK YOUR ANSWERS TO THE LAST GROUP OF QUESTIONS.

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2 (a) No. There is no movement of any material through the wall of the coffee cup. It is characteristic of heat conduction processes that there is no large-scale movement of matter. (b) D is correct. Since the cup will be at a uniform temperature the violence of oscillation (which is a measure of the temperature) will be the same throughout the thickness as represented by:

(c) B is correct. The inside regions of the cup will be hotter than the outside and the atoms there will be oscillating more violently.

(d) A is correct. During the first minute the temperature of the outside of the cup will increase. The atoms there will oscillate more violently.

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INTERLUDE. ATOMIC PROCESSES IN CONDUCTION The answers to the previous questions suggest that the transfer of heat energy from the inside surface of the cup to the outside has taken place by the transfer of mechanical oscillation energy of the atoms without any large scale movement of the atoms themselves. The atoms remain tied to their "home bases". It is easy to see how this transfer of energy can occur. A violently oscillating atom can "nudge" (push or pull) a neighbouring atom via its interatomic bond (spring) making this atom oscillate more violently. Imagine the atoms on the inside being shaken vigorously. Energy would be transmitted through the chains of atoms to the outer surface. The transfer of heat energy in solid materials by the process of conduction involves the transfer of mechanical energy (of oscillation) from one atom to another. In liquids energy is similarly handed on from one atom to the next. An atom "jostles" its neighbours and hands on some of its kinetic energy. In this way heat energy can be transmitted through a liquid. (We will soon see that in liquids there is another very powerful process of heat transfer-Convection.) In the next question we consider heat conduction in gases. 3. In gases the conduction of heat also involves the transfer of mechanical energy by the constituent atoms or molecules. Imagine a box of gas with the molecules flying around freely inside. If one wall of the box is heated, molecules striking this wall can pick up some extra energy from the violently oscillating atoms in this wall. What form is this energy most likely to take? A Kinetic energy B Potential energy C Chemical energy D Electrical energy ANS After rebounding from the wall the molecules carry this energy and transfer it to other molecules or maybe to the opposite wall (if they don't meet other molecules on the way). This process is occurring very frequently and the overall result is the transfer of energy from the hot wall to the cold wall i.e. heat is conducted through the gas. CHECK YOUR ANSWER TO THIS QUESTION.

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3. A. The extra energy is predominantly in the form of kinetic energy (½mv2). Since the molecules are free, potential energy is not involved. Electrical and chemical effects would be negligible unless the hot wall was VERY HOT.

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4. Try to explain why gases are generally much poorer conductors of heat than solids. For instance the thermal conductivity* of water vapour or air is about one-thousandth of the thermal conductivity of ice. (Hint: The density of ice is many hundreds times the density of air or water vapour. Think about the problem in terms of the behaviour of atoms or molecules.) Explanation: * This is a measure of the amount of heat conducted through a slab of material with a given temperature difference maintained across its thickness. It will be described in detail later.

CHECK YOUR ANSWER TO THIS QUESTION.

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4. The simplest explanation is that there are many more molecules or atoms transferring the energy in a solid. A slab of ice will contain a thousand tines more H2O molecules than a slab of water vapour of the same size. Since there are many more carriers of energy in ice the transfer process is more efficient than in a gas.

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5. Recall the example of heat being transferred from the hot coffee through the cup to your hand. (a) If the cup were made of aluminium instead of china you would expect that your hand

would receive heat ................................... (more rapidly, more slowly, at the same rate). (b) You would describe aluminium as being a .................................... (better, worse, similar)

heat conductor compared with china. CHECK YOUR ANSWERS TO THIS QUESTION. Solid materials have a wide range of thermal conductivities. Copper and silver conduct heat a thousand times more effectively than glass. 6 (a) As well as copper and silver other good thermal conductors are aluminium, gold, iron

and platinum. These materials are all of a class known as ..............................(metals, ceramics, polymers, insulators).

(b) At the atomic level these materials are characterised by the presence of free

...............................(ions, atoms, electrons, molecules). CHECK YOUR ANSWERS TO THIS QUESTION. These electrons are able to move freely through the regular structure of ions (atoms which have lost one or more electrons). The electrons can move freely rather like the molecules in a gas and are able to carry (kinetic) energy from the hot regions to the cold regions. 7 In a metal therefore, there are two ways in which heat can be transferred, one by the oscillating ions and the other by the ...................................(electrons, vibrating atoms, interatomic bonds, freely moving molecules). CHECK YOUR ANSWER TO THIS QUESTION. 8. There are a large number of electrons taking part in the energy transfer process. The transfer of energy by these freely moving electrons is much more efficient than the transfer by the oscillating ions. It is for this reason that metals which contain free ....................................... are much better heat conductors than those materials (e.g. glass, china) which do not contain free electrons but rely solely on energy transfer by the ..................................................... ( oscillating atoms, stationary atoms, freely moving atoms, freely moving molecules) CHECK YOUR ANSWER TO THIS QUESTION.

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5 (a) more rapidly. Your hand would "feel hot" much sooner after coffee was poured into an aluminium cup. (b) Aluminium is a better heat conductor. Alternatively china could be described as a better thermal (or heat) insulator than aluminium. 6 (a) metals (b) Metals are characterised by the presence of free electrons. The ions (atoms which have lost one or more electrons) are certainly not free. They are "tied to" and oscillate about their "home base". Molecules (groups of atoms of different types) are not present in a pure metal. 7 electrons Apart from the ions (interconnected by their bonds) the only other entities available to carry energy are the free electrons. 8 Metals contain free electrons. Glass and china rely on energy transfer by the oscillating atoms. The response "stationary atoms" is incorrect. Atoms or molecules in materials whether solid, liquid or gas are always in motion (even at the absolute zero of temperature). Atomic motion never ceases and as explained earlier the temperature is closely related to the violence of this motion. The responses referring to freely moving atoms or molecules are incorrect. These situations are typical of a gas.

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Our discussion of thermal conductivity to date has been concerned with processes at the atomic level and has been qualitative. It is of course necessary to make numerical estimates of how much heat energy is conducted through a piece of material of given size and shape. Such estimates would be made in the calculation of heat loss from your house in winter. 9. To explore this, consider a thin slab of material of thickness L and area A as shown. One face of area A is maintained at temperature T1 and the opposite face at temperature T2. The amount of heat energy transmitted per second from one face to another would be expected to depend on some of the following quantities. Tick these quantities

The area A. The thickness L. Not on A and L independently but only on the volume of the slab AL. Only on T1 but not on T2. Only on T2 but not on T1. Only the average temperature ½(T1+T2). Only the temperature difference (T1-T2). The particular material of which the slab is made.

. CHECK YOUR ANSWER TO THIS QUESTION. 10 The relation between the quantities discussed in the previous equation is a particularly simple one. It is that the heat flow per second H is given by:

Simple relations like this are commonplace in Physics, for example, relations of a similar form govern the flow of electricity in a wire or the flow of water in a pipe. From the expression for H we can see that, if we keep other things constant, and double the area, the heat flow per second will................(halve, double, stay the same...?), or if we double the temperature difference, the heat flow will .................., or if we double the thickness the heat flow will ..................... CHECK YOUR ANSWER TO THIS QUESTION.

)T(TLA .constant H 21 −=

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9. The following quantities should have been ticked The area A The thickness L Only the temperature difference (T1-T2). The particular material of which the slab is made The following quantities should not have been ticked Not on A and L independently but only on the volume of the slab AL Consider two pieces of the same material (i) a thin slab 1m.by 1m.by 1mm. thick (ii) a cube 10cm.by 10 cm.by 10cm . Both will have the same volume but for the same temperature difference between

opposite faces the thin slab, because of its large surface area and small thickness will conduct much more heat per second than the cube--actually ten thousand times more.

Only on T1 but not on T2. If T2 were different from T1 there would be a flow of heat, but if T2 were the same as T1 the slab would be at a uniform temperature and no heat would flow through it i.e. the heat flow does not depend only on T1.

Only on T2 but not on T1. The same argument applies On the average temperature ½(T1+T2). Consider two identical slabs: Slab (i) has T1=100°C and T2=0°C Slab (ii) has T1= 50°C and T2= 50°C

Both slabs have the same average temperature (50°C) but slab (i) will be conducting while slab (ii) will not because it is at a uniform temperature.

10 double double halve

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11 The relation shows how H depends on the geometry (size and shape) of the slab and the temperature difference. The constant of proportionality depends only on the particular material. It is known as the thermal conductivity and we will denote it by the letter k. Thus

This is the simple law of heat conduction. It is necessary that the quantities appearing in this relation have consistent units. If we use S.I.units (Système Internationale d' Unités) then the units of (a) H the heat energy flow per second will be in ................................... (b) A the area will be in ......................... (c) L the thickness will be in ................... (d) T1 and T2 the temperatures will be in ............... (e) Use the equation for thermal conduction to determine the correct S.I. units for the thermal conductivity k. ANS to (e).................... CHECK YOUR ANSWER TO THIS QUESTION.

)T-T(LAk.=H 21

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11. (a) Joule per second, J s-1 , or watt represented by the letter w. The Joule is the unit of energy which you used in earlier studies of specific and latent heats. The unit watt, is a special unit for measuring energy per second or power. 1 watt is equal to 1 Joule per second. Your electrical appliances are rated in watts, the electrical energy per second which is fed into them to be converted to some other form. A small electric radiator is typically 1000 watt while a fluorescent light tube is typically 20 watt. (b) square metres or m2 (c) metre (d) degrees Celsius, °C or degrees Kelvin, K Degrees Kelvin are, strictly speaking, the correct S.I. unit, but degrees Celsius are in

common use. The temperature intervals 1°C and 1K are the same but on the Kelvin scale the temperature of melting ice (1°C) is approximately 273K.

(e) k is given by the relation

The units are therefore

ANS to (e) is:

)T-TA(

LH=k21

C m w=C ms J=C metre sec

Joule=C )(metre

metresec

Joule 1-1-1-1-1-oo2 °°

C m w -1-1 °

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12. Some typical values of thermal conductivity are:

Material Thermal Conductivity k w m-1 °C-1

Aluminium 200

Lead 35

Glass 1.0

Water 0.6

Building Brick 0.6

Concrete 0.1

Air 0.02 Using these values answer the following questions. (a) Which would be more effective in reducing the heat loss from a building, a single brick wall 12cm. thick or a concrete wall 4 cm. thick? Make a numerical calculation comparing the heat flow in the two cases. ANS.......................... (b) A house window of thickness 5 mm. is to be removed and the hole bricked up with bricks of thickness 12 cm. Will the heat loss through this area be increased or decreased? By what numerical factor? ANS (c) Air is a very good thermal insulator (a bad thermal conductor). List three cases where this is made use of. CHECK YOUR ANSWERS TO THIS QUESTION.

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12. (a) Concrete is more effective. It follows from the formula for heat conduction that for the same wall area and temperature difference

The heat flow through the brick wall is twice that through the concrete wall. Thus concrete is more effective at reducing heat loss. (Note that since a ratio of thicknesses is involved in the calculation it was not necessary to convert these to metres. All that is required is that they are in the same units (cm. in this case).) (b)The heat loss will be decreased. It will be reduced by a factor of 1/40.

The heat flow has been reduced to one-fortieth of its previous value i.e. by a factor of 1/40. Note that in this case the window thickness had to be converted to the same units as the brick thickness (cm. in this case) before substitution in the formula. (c) Some commonplace examples are: The air space in cavity walls in buildings. The air trapped in wool used in blankets. The small air cells in foam plastic cups. Your examples should refer to situations where air is in some way confined to a limited space. If it is not confined then large convection currents can be set up and the useful low conductivity property of air is lost. The insulating property is lost because of the occurrence of convection. This process is described in the next section.

2=0.14

120.6=

)k(concrete)L(concrete

L(brick)k(brick)=

concrete through flowHeat brick through flowHeat

××

401=

1.00.5

120.6=

k(glass)L(glass)

L(brick)k(brick)=

glass through flowHeat brick through flowHeat

××

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13 Usually, when a liquid is heated its volume will increase (a) Thus, when it is heated its density will A increase B decrease C remain the same. ANS..... (b) If heat is added locally to a small portion V of liquid, located at the bottom of a larger volume of the same liquid, this portion would A rise B remain in the same place C spread across the bottom of the container. ANS....... CHECK YOUR ANSWERS TO THIS QUESTION. This process forms the basis of an important method of heat transfer known as CONVECTION. If heat energy is injected locally into a fluid (a liquid or a gas) the fluid will expand and rise. Cool fluid will move in to take its place and this will in turn expand and rise. The effect of all this is a continuing circulation of the fluid carrying heat energy with it. This is a very efficient means of heat transfer. Although air is a poor conductor of heat it can transfer heat very efficiently if it is free to move. An example of this is the warming of a room by the air set in circulation by an electric heater as shown in the diagram where the electric space heater is providing the local heating. This process can be assisted by forcing the air to circulate as with a small fan heater. 14 Can a space-man in zero gravity make use of convection processes to warm his space-craft? Give, if possible detailed reasons to support your answer. CHECK YOUR ANSWER TO THIS QUESTION.

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13 (a) B. The volume of a given mass of liquid has increased, therefore its density (mass ÷ volume) will decrease. (b) A. The portion of the liquid will rise. A body of lower density than the fluid in which it is immersed will rise. Commonplace examples are a piece of wood or an ice cube in water or a helium-filled balloon in air. 14 No. In zero gravity there is no convection. In a gravitational field such as that on earth a body will rise if immersed in a fluid of higher density. This buoyancy is a result of pressure forces acting on the body. The pressures are in turn, a result of gravitational forces acting on the fluid and the body (their weights). Where there is no gravity there is no weight and no buoyancy force. The same considerations apply to a less dense liquid in a higher density liquid. In a gravitational field the less dense liquid will rise because of buoyancy forces. Where there is no gravity there is no buoyancy force and no convection. Many of the experiments carried out in SKYLAB were to investigate various processes in the absence of convection, for example the cooling and solidification of liquid metals and alloys.

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15 (a) When warmed by the sun which of the following is the principal means by which heat energy is carried to us from the sun? A A stream of atoms B A stream of electrons C Radiation D Hot gas ANS (b) Which of the following processes are involved in this? A The movement of bulk matter as in convection. B Energy being passed from one oscillating atom to the next. C Energy being handed on from one molecule to the next during collisions. D None of these. ANS CHECK YOUR ANSWERS TO THIS QUESTION. Heat transfer by radiation is commonplace. Everything emits heat radiation whether it is hot, warm or cold. The radiation travels outward from the body and behaves much like light although we can't see it i.e. our eyes are not sensitive to this heat (infra-red) radiation. 16 Since it is similar to light heat radiation would be expected to pass through a vacuum gases liquids solids (Tick your choices) GO STRAIGHT TO THE NEXT QUESTION. DON'T CHECK YOUR ANSWER YET. WAIT UNTIL YOU ARE INSTRUCTED TO DO SO. On striking something, radiation may be absorbed, transmitted or reflected, usually some of each. 17 If the heating effects of radiation on a body are to be as effective as possible it should be mostly absorbed reflected transmitted (Tick your choice) CHECK YOUR ANSWERS TO THE LAST GROUP OF QUESTIONS. REFER TO THE APPROPRIATE SECTIONS OF YOUR TEXT BOOK FOR FURTHER INFORMATION.

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15 (a) C is correct. The carrier of heat energy is radiation which is somewhat similar to light but of a longer wavelength than red light. It is known as infra-red radiation. We cannot see it but it makes its presence felt by warming us. Although particles are sent out by the sun they are not important in warming us. (b) D is correct. The movement of atoms,molecules or any form of matter is not involved in transfer of energy by radiation. 16 All four answers are correct. Radiation will certainly pass through a vacuum. Light and heat radiation reaching us from the sun pass through the near perfect vacuum in space. Radiation will pass through any solid, liquid or gas. Light will pass through glass and can be observed to pass through metal if it is thin enough. 17 Absorbed is the correct response. It is only radiation which is absorbed by the body which is converted into heat energy inside the body. Radiation reflected from the body or transmitted through it does not help to heat the body.

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18 The amount of infra-red radiation given off by a heated body depends very strongly on its temperature. In fact, the total energy emitted per second is proportional to T4 where T is the absolute temperature measured in K (kelvin). All bodies are at absolute temperatures greater than 0K. Therefore they will all emit some heat radiation. When two bodies I and II are involved in the transfer of radiation they are both emitting and receiving heat. For body II to be heated by body I as quickly as possible (i.e. to increase its internal energy as quickly as possible) (a). I should be as……..( hot or cold) as possible. (b) II should be emitting as ……..(much or little) heat energy as possible (c) II should be absorbing as……..(much or little) heat energy as possible (d). II should be reflecting as……..(much or little) heat energy as possible GO STRAIGHT TO THE NEXT QUESTION. DON'T CHECK YOUR ANSWER YET. WAIT UNTIL YOU ARE INSTRUCTED TO DO SO. 19 (a) A car parked in the sun "heats up". To minimise the effect the car should A absorb as much heat as possible B absorb as little heat as possible C emit as much heat as possible D emit as little heat as possible E reflect as much heat as possible F reflect as little heat as possible ANS...... (b) Which of A-F above is appropriate to a solar panel which is part of a domestic hot water system? ANS..... GO STRAIGHT TO THE NEXT QUESTION. DON'T CHECK YOUR ANSWER YET. WAIT UNTIL YOU ARE INSTRUCTED TO DO SO.

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18 . In answering this question you should have realised that the energy falling on body II must be either absorbed, reflected or re-emitted i.e. Incident energy = absorbed + reflected + re-emitted Thus if the energy absorbed by body II is to be as high as possible then (a). I should be as hot as possible so that the energy incident on II is as large as possible. (b) II should be emitting as little heat energy as possible (c) II should be absorbing as much heat energy as possible (d). II should be reflecting as much heat energy as possible 19 (a) B,C and E are the correct responses. It is required that the car "takes up" the smallest amount of heat energy possible i.e. it must absorb a small amount (B) and reflect a large amount (E). Of the heat energy absorbed it should get rid of as much of this as possible by emission (C). (b) A,D and F are the correct responses. The requirements here are just the opposite of those of the previous problem. The panel should take up as much energy as possible, absorbing much (A), reflecting little (F) and getting rid of as little as possible by emission (D).

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20 A simple electric radiator consists of an electrically heated "element" with a shiny curved metal sheet behind it (a) The main function of the element is to A emit light B re-direct the heat from the metal behind it. C emit infra-red radiation D conduct heat from one end to the other. ANS........ (b) The main function of the curved metal sheet is to A absorb the radiation from the element B reflect the radiation from the element in the forward direction C assist the flow of air by convection D conduct heat away from the air behind the element. ANS........ CHECK YOUR ANSWERS TO THE LAST GROUP OF QUESTIONS. REFER TO THE APPROPRIATE SECTIONS OF YOUR TEXT BOOK FOR FURTHER INFORMATION. 21 The left hand column contains a number of questions relating to processes of heat transfer. Answer these questions with a "Yes" or "No" in each of the three columns and make a short comment to justify your answer.

Convection Conduction Radiation

a) Is large scale movement of bulk matter involved in this process ?

b) Is molecular or atomic movement involved in this process ?

c) Can this process transfer heat energy through a vacuum?

d) Can this process transfer heat energy through a solid?

e) Is this the main means of transferring heat through the tiles of your house roof?

f) Is this the main means of distributing heat in the water in an electric kettle?

CHECK YOUR ANSWERS TO THIS QUESTION.

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20 (a) C is the correct response. A is incorrect. Although some light is emitted the main function of the element is to emit infra-red (heat) radiation. D is incorrect. Some heat would be conducted along the element (towards the ends) but this would detract slightly from the operation of the radiator. It would tend to lower the temperature of the element. B is incorrect. It is, in fact, the function of the metal reflector to redirect radiation emitted by the element into the forward direction. (b) B is correct. The metal is shiny so that it is an efficient reflector. A rusty piece of steel would be much less effective. A and D are incorrect. The metal is not there to absorb radiation or conduct heat. C is incorrect. Although there is some convective flow of air from near the element, the curved metal sheet is not designed to assist this. From your knowledge of reflection of light from curved mirrors you should be able to describe the type of curve used for the reflector and the location of the element with respect to this. 21

Convection Conduction Radiation

a Yes. This is the very essence of convection.

No. Although atoms or molecules move there is no movement of bulk matter

No. Radiation does not rely on the presence of matter

b Yes. Since bulk matter is moving, atoms are certainly moving

Yes. The moving atoms or molecules are the carriers of energy

No. See above.

c No. The presence of matter is necessary.

No. The presence of matter is necessary.

Yes. For example from the sun.

d No. The material must be capable of large scale movement. Only a liquid or gas will do.

Yes. As in the case of heat passing through a coffee cup.

Yes. The sun's radiation will pass through a window.

e No. See above. Yes. This is similar to the coffee cup situation.

No. Their upper surface is heated by radiation. Transfer through the tile is by conduction.

f Yes. No. It would make only a small contribution.

No. It would make only a small contribution.

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22 Light radiation is emitted in all directions from a candle flame as you can easily observe. Which of the following statements is true for heat radiation? A No heat radiation is emitted from the candle flame. All the heat coming from the flame is transferred by convection. B Heat radiation is emitted in all directions. It behaves like light. C Heat radiation is only emitted upwards from the flame. Your hand gets hot if placed above the flame. D There is no heat radiation from the flame. The candle would not burn in a vacuum. ANS........ Comment on the other answers. GO STRAIGHT TO THE NEXT QUESTION. DON'T CHECK YOUR ANSWER YET. WAIT UNTIL YOU ARE INSTRUCTED TO DO SO. 23 You have just taken two cold cans of Coke out of the refrigerator (same size, same temperature) and left them on the kitchen table, one wrapped in aluminium foil and the other in a woollen scarf. Which of the following describes the subsequent happenings? A The can in the woollen scarf will warm up fastest because wool is warm and supplies heat. B The can in the aluminium foil will stay cold longest because metals hold heat (or cold) better. C The can in the aluminium foil will stay cold longest because the aluminium foil will reflect heat away. D The can in the aluminium foil will warm up fastest because the aluminium conducts heat into the can more efficiently than the scarf. ANS....... Comment on the other answers. GO STRAIGHT TO THE NEXT QUESTION. DON'T CHECK YOUR ANSWER YET. WAIT UNTIL YOU ARE INSTRUCTED TO DO SO.

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22 B is correct. A is incorrect. All three types of heat transfer (conduction, convection and radiation) contribute to the heat transfer from the flame. C is incorrect. Your hand certainly gets hottest if held above the flame because it is here that the transfer by convection is most obvious. Radiant heat can be felt by the finger tips at the side of the flame. D is incorrect and is a very "mixed up" statement which confuses two entirely different ideas: (a) the passage of radiation through a vacuum (b) a candle will not burn in a vacuum because of lack of oxygen. 23 D is correct. Here we are concerned (mainly) with the conduction of heat from the outside air to the cold can and this conduction is more efficient through the aluminium foil. The metal aluminium is a good conductor of heat whereas a woollen scarf with included pockets of air is a good thermal insulator. A is not correct. Wool keeps us warm because it allows only a small amount of heat to escape from our body. It certainly does not supply heat. B is not correct. The statement that "metals hold heat better" is not particularly meaningful. A copper block would in fact cool more rapidly than a glass block of the same size and shape and at the same initial temperature. This is in fact a complex problem. You may like to think about the quantities which are involved. C is incorrect. It refers to radiant heat energy which is falling on the wrapped cans. The amount of radiant energy coming from the walls and other objects in the kitchen would be quite small in comparison with the heat transferred from the surrounding air. Convection takes place near cold objects as well as near hot ones. Warm air will continually move in to replace the air which has been cooled i.e. given up its heat to the scarf or foil.

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24 Once a month we see a full moon. At other times we see only a part of the moon's surface illuminated. Does the dark side of the moon emit heat radiation? Give your reasons. GO STRAIGHT TO THE NEXT QUESTION. DON'T CHECK YOUR ANSWER YET. WAIT UNTIL YOU ARE INSTRUCTED TO DO SO. 25 You are given the task of making cylindrical lead bullets by casting which involves pouring molten lead into a cylindrical cavity in a block of some material (called a mould). You wish to cool the lead as quickly as possible. Which material would you choose for the mould? A Copper B Wood C Glass D Ice CHECK YOUR ANSWERS TO THE LAST GROUP OF QUESTIONS. REFER TO THE APPROPRIATE SECTIONS OF YOUR TEXT BOOK FOR FURTHER INFORMATION.

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24 Yes. Heat radiation is emitted by the dark side of the moon. The dark side of the moon is shaded from the sun and does not reflect light or heat radiation from the sun. BUT the dark side of the sun is at a finite temperature (approximately - 170 °C or 100K) and will therefore emit some heat radiation. Although not emitting perceptible visible light, cold objects still emit heat (infra red) radiation. Night spying devices rely on detecting heat radiation from bodies. 25 A. Copper would be the best choice. Here the aim is to get the heat away from the lead as quickly as possible. All three modes of heat transfer will take place but radiation and convective loss in the air above will play only a small part. Most of the loss occurs by thermal conduction through the mould. Copper is then the best choice of material since it is the best heat conductor. If you were tempted to use ice (response D) remember that your task was to make cylindrical bullets. The molten lead would immediately melt some of the ice and the resulting piece of lead would be far from cylindrical.

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